As indicated in the Lenny (oldstable) release notes, security support for
the Icedove packages in the oldstable needed to be stopped before the end
of the regular Lenny security maintenance life cycle.
You are strongly encouraged to upgrade to stable or switch to a different
mail client.

For the stable distribution (squeeze), this problem has been fixed in
version 3.0.11-1+squeeze5.

For the unstable distribution (sid), this problem has been fixed in
version 3.1.15-1.